In recent years, the industry has been undertaking an intensive effort in designing, developing and manufacturing power plants that exhibited a high trust specific fuel consumption (TSFC). To this end it is desirous to operate the engine during at least a portion of its operating envelope at a temperature higher than has heretofore been achieved. One of the consequences of these "hotter" engines is that the mid-frame bearings supporting the rotating machinery is disposed in a hotter and higher pressure environment than similar sections in other engines have heretofore been encountered.
Coincidental with disposing the bearings in a hotter environment, outside bearing compartment presssures have risen attendantly. The consequences of these conditions, namely hotter and higher pressure environment and pressurized bearing compartment could result in an undsirable condition with the bearing compartment, which many manifest in fires and degradation of the components.
While the industry has provided buffer means for bearings in heretofore known engines, such systems flow air at a lower temperature and much lower pressure and/or cooler air at the same pressure into a cavity surrounding the bearing or in close proximity thereto. For such heretofore known systems to work satisfactorily it is necessary to continuously flow the air to lower pressure areas of the engine in order to generate sufficiently lower pressure at the seal adjacent the bearing compartment. In some installations it was also necessary to vent the excess flow overboard. Whether that air is being vented overboard or routed to a lower pressure area, by bypassing the high pressure turbine, the use of the air in this manner results in an overall loss in engine performance. To obtain the proper pressure in these heretofore designs it is necessary to create a pressure drop by the use of seals and in practice these seals are typically of the labyrinth design. Obviously, the constant wear of these seals adversely affect the pressure drop valve and hence flow.
Thus, these heretofore known systems incur disadvantages requiring a substantial amount of air to buffer the external surfaces of the bearing compartment and discharging the buffer air to lower ending pressures or overboard and requiring pressure reducing seals.
This invention serves to solve the particular problem enumerated above by providing a high pressure, cooler air buffer zone for a bearing that is encapsulated adjacent to the combustion section. We have found that we can obtain a satisfactory environment that minimizes the likelihood of inadvertent fires in the area of the bearings by applying adjacent to the seals of the bearing compartment high pressure compressor air that has been cooled to some temperature below the flash point of the oil/air mixture associated with the bearing cooling and lubrication system. The pressure of the buffer air is slightly higher than the hotter ambient air so that the hotter air will not migrate to the bearing compartment in the event of the seal malfunctioning. It is contemplated that the cooled compressor bleed air is fed to a buffer zone formed adjacent to the carbon seals of the bearing compartment. The buffer zone is defined by an elongated annular space defined between the rotating shaft and a stationary annular, radially spaced, parallelly disposed extension number. This member being open ended so that flow admitted to said buffer zone intermediate the ends thereof flows axially in either direciton with a preference of flowing toward the radial seal disposed on one of said ends.
In one of the preferred embodiments the buffer zone is fed by at least one pipe (preferably two) for each of the buffer zones associated with the pair of carbon seals. In another embodiment the bearing compartment is shrouded defining an annular passage feeding the buffer zone, which passage cools the entire bearing compartment surface. The second outer shroud forms a heat shield by surrounding the annular passage and is dead-ended to provide an insulation of dead air.
It is contemplated by this invention that the air bled from the compressor to buffer the bearing compartment is utilized downstream of the engine in a manner that will not adversely penalize the engine's performance.
Notwithstanding the fact that the compressor air routed to the bearing compartment has been heated to some degree in its contact with the combustor environment, the air still is sufficiently cool to be useful in cooling the hot high turbine rotor. And since the pressure of this air is sufficiently high to be compatible with the pressure in the high turbine environment, it is routed there to scrub this hot section. In engines that employ tangential on board injectors (TOBI) for cooling the high turbine rotor, the advantages of utilizing this invention results in utilizing less TOBI air than would otherwise be necessary. It is possible, because of the relative low temperature of the air from the buffer to the TOBI air, a significant savings in the use of the overall cooling air for the high pressure turbine may be realized. For a complete description of a TOBI refrence should be made to U.S. Pat. No. 3,768,921 granted to W. M. Brown and W. A Grace on Oct. 30, 1973 and assigned to the same assignee as this patent application.